Dynamic range control system having amplitude restoration

ABSTRACT

An automatic dynamic range control system that eliminates the effect of dynamic range limitations in the main radar channel and restores the signal amplitude prior to detection. A separate auxiliary measurement channel is provided in parallel with the main IF channel having a logarithmic amplifier or sequential detector with sufficient dynamic range to meet the signal informational requirements. During each range bin period, the IF signal in the auxiliary channel is quantized in amplitude above the predetermined dynamic range of the main channel within selected ranges of input signal level to form coded signals. The quantized levels are then utilized substantially in real time for attenuating the signal in the main IF channel to maintain that signal within the dynamic range limits of the channel. After the main channel signal is processed and passed through an analog to digital converter, the coded signal for each corresponding range bin is utilized to recombine a component to the attenuated signal so that the radar signal has its original amplitude modulations restored. Thus, the system allows reliable radar detection while at the same time overcoming the limitations of limited gain characteristics.

United States Patent Eifinger *et al.

[ DYNAMIC RANGE CONTROL SYSTEM HAVING AMPLITUDE RESTORATION Inventors:David D. Effinger, La Habra; Norol T. Evans, San Pedro; Vaughn H.Estrick, Fullerton, all of Calif.

Primary Examiner-Benjamin A. Borchelt Assistant Examiner-G. E. MontoneAtt0rney-W. I-I. MacAllister 'iir'nfn'g Control Dec. 25, 1973 [57]ABSTRACT An automatic dynamic range control system that eliminates theeffect of dynamic range limitations in the main radar channel andrestores the signal amplitude prior to detection. A separate auxiliarymeasurement channel is provided in parallel with the main IF channelhaving a logarithmic amplifier or sequential detector with sufficientdynamic range to meet the signal informational requirements. During eachrange bin period, the IF signal in the auxiliary channel is quantized inamplitude above the predetermined dynamic range of the main channelwithin selected ranges of input signal level to form coded signals. Thequantized levels are then utilized substantially in real time forattenuating the signal in the main IF channel to maintain that signalwithin the dynamic range limits of the channel. After the main channelsignal is processed and passed through an analog to digital converter,the coded signal for each corresponding range bin is utilized t0recombine a component to the attenuated signal so that the radar signalhas its original amplitude modulations restored. Thus, the system allowsreliable radar detection while at the same time overcoming thelimitations of limited gain characteristics.

10 Claims, 6 Drawing Figures 94 I00 c C l g 122 1 Log 4 A 2O Amplifier DEncode Dewy /23 IE TR .c 106 C l4 '6 I8 96 Y Y J 5 rm r upex 2 R i lVldeO mm W y at/ f e ,1 oc Umt'i REC PS 36 so 82 23' UL 78 84 86 REF.1.o,4.......J 30 39 2 44 L- 1 D-MW|M "W7 m I I u 64 6| 5 78 38 Atten 0/6db Anen Men F r A T FF-I IF I Q O/IZ db 0/18 db. D Dewy 'l l DET "z" IMT 60 l 67 as J I 37 12 E l l 1 H4 Decode Logic I. DYNAMIC RANGE CONTROLSYSTEM HAVING AMPLITUDE RESTORATION BACKGROUND OF THE INVENTION 1. Fieldof the Invention This invention relates to radar automatic gain controlsystems and particularly to an automatic dynamic range control systemthat overcomes the dynamic range limitations of a radar channel withoutsubstantially affecting the target detection characteristics.

, 2. Description of the Prior Art Radar systems utilizing MTI (movingtarget indication) or monopulse processing may have dynamic rangelimitations in the main IF channel such as in the analog to digitalconverters which may clip the signal to cause a loss of targetinformation. If a conventional instantaneous automatic gain controlsystem is utilized, the target amplitude information may be lost, thusresulting in unreliable target detection in the video detection portionsof the processor. If a two lobe monopulse sequence in either azimuth onelevation dimension is utilized, the amplitude must be maintained in thedimension in which the sequential lobing is being performed. Aconventional instantaneous automatic gain control system substantiallysuppresses or limits this information so that target boresight cannot beaccurately detected. Thus radar systems which have either clipping orlimiting of the signal in the main channel may have a considerabledegree of inaccuracy and unreliability in its target detection andtracking capabilities.

It is therefore an object of this invention to provide an improved radarsystem that substantially eliminates the undesired effects of a limiteddynamic range in a radar channel.

It is another object of this invention to provide a large dynamic rangeradar system that retains for the detection operation, a substantialportion of the information contained in the signal.

It isa further object of this invention to provide an automatic gaincontrol for radar systems that eliminates the effects of limiting orclipping of the signal while at the same time providing signal amplitudecharacteristics required for the signal processing.

It is a still further object of this invention to provide an improvedradar system utilizing a moving target indicator.

It is still a further object of this invention to provide an improvedradar system having a desirable monopulse oeration.

BRIEF DESCRIPTION OF THE DRAWINGS principles of the invention;

FIG. 2 is a schematic block diagram of another arrangement ofa portionof the system of FIG. 1 in accordance with the invention;

FIG. 3 is a schematic block diagram of still another arrangement of aportion of the system of FIG. 1;

FIG. 4 is a schematic block diagram of an analog to digital convertersystem that may be utilized in the system of FIG. 1;

FIG. 5 is a schematic diagram of waveforms showing time as a function ofdynamic range for explaining the operation of the system of theinvention; and

FIG. 6 is a schematic diagram of waveforms showing voltage as a functionof time for further explaining the signal amplitude restorationoperation of the system of the invention.

DETAILED DESCRIPTION OF THE INVENTION The system of the inventionincludes an auxiliary channel 10 utilized to control the gain in a mainchannel 12 illustrated as an IF channel but which in some arrangementswithin the scope of the invention may be at other frequencies such as RF(radio frequency). The radar system includes an antenna 14 which may bea planar array or a parabollic dish, for example, and an antenna controlcircuit 16 which may include micro wave connections and scan controlunits coupled to a suitable duplexer 18, which as is well known in theart, is then coupled to a transmitter 20 and receiver 22. Pulses ofenergy indicated by a waveform 21 are transmitted into space from thetransmitter 20 in response to a timing control circuit 26, and energyintercepted by the antenna is then applied to the receiver 22 asindicated by a waveform 23. The signal is converted to an IF(intermediate frequency) signal in a conventional manner in the receiver22 and applied through a lead 30 to a suitable power splitter 32. The IFsignal on the lead 30 may, for example in the illustrated system, have adynamic range requirement of 74 db and the channel 12 may have a limiteddynamic range of, for example, 50 db. Dynamic range is defined as theratio of the specified maximum signal level capability of the system orcomponents to its noise level. Each signal has dynamic rangerequirements for providing a desired amount of amplitude information.Thus, dynamic range which may be expressed in decibels is a measure ofthe value of the S/N ratio over which a system or component can operate.

The IF signal is then applied from the power splitter 32 through a lead36 to an analog IF delay unit 37, which for example may be a delay line,and is then applied through a lead 38 to an attenuator unit 39.Attenuators 40, 42 and 44 may be included in the unit 39 coupled inseries and respectively providing attenuations ofO or 6 db, ofO or 12 dband ofO or 18 db. The attenuator 40, for example, includes a powersplitter (PS) 48 coupled to the lead 38 to apply a signal through a 6 dbattenuator 50 when a switch 54 is in a first position to conduct througha lead 56, or to apply a signal without attenuation through the switch54 to an output lead 58 when the switch is in a second position. Thelead 58 then passes a signal through an attenuator unit 42 which is ofasimilar configuration which in turn passes a signal through a lead 60 tothe attenuator unit 44 also of a similar configuration.

The signal is then applied from the attenuator unit 44 through a lead 64to a phase detector 61 which also receives a reference local oscillator(LO) signal from the receiver 22 to apply a video or envelope signalthrough a lead 67 to an analog to digital (A/D) converter 66. Some ofthe dynamic range limitations in the channel I2 may be provided by theanalog to digital converter 66. The digital signal is then applied fromthe converter 66 through a lead 68 to a recombiner unit 72 which in theillustrated system with a linear signal passing through the channel 12and a logarithmic signal passing through the auxiliary channel 10, is adigital multiplier. An MTI unit (moving target indicator) is shown by adotted box 78 coupled in the lead 68 for a. system in which movingtarget indication is provided. The signal at the output of therecombiner 72 is then applied through a lead 80 to a detector 82 and inturn to a video processor 84 which applies signals to suitableutilization units 86 which, for example, may be display units.

The auxiliary channel includes a logarithmic amplifier 94 or sequentialdetector, receiving the IF signals through a lead 96 from the powersplitter 32. As is well known in the art, logarithmic amplifiers orsequential detectors utilize successive detection over predeterminedranges of gain to provide a wide dynamic range of operation. Asequential detector that may be utilized in the system of the inventionis shown, for example, on pages 5-34 of the book, Radar Handbook byMerril I. Skolnik, copyright 1970 and published by McGraw Hill BookCompany. In the illustrated system the logarithmic amplifier 94 has adynamic range greater than 74 db, which is the dynamic range requirementof the IF radar signal. It is to be noted that the principles of theinvention are not to be limited to any particular logarithmic amplifierbut include any suitable amplifier having sufficient dynamic range todetect the total amplitude of the radar signal. The amplified signal isthen applied from the logarithmic amplifier 94 through a lead 100 to ananalog to digital converter 102 which encodes the signal to valuesrepresenting ranges above the dynamic range limitations of the mainchannel 12. The digital codes are then applied through a composite lead106 to an encoding unit 108 and in turn through a suitable lead 110 to adecode logic unit 114. In response to'the encoded signal the decodelogic unit 114 then controls the switches of the series coupledattenuators 40, 42 and 44 to provide the desired attenuation. Forrecombining the attenuated amplitude portion of the signal, the encodecircuit 108 applies a coded signal through a lead 120 to a delay unit122 and in turn to the multiplier 72 in the illustrated arrangement inwhich the signal in the channel 12 is a linear signal. A clock 130 isprovided to apply signals C to the various digital units such as 102,108 and 72 to apply a clock signal to a divide circuit 132 which in turnapplies signals to the timing control circuit 26 for controlling thetransmitter sequence.

Referring now also to FIG. 2, the recombiner unit is shown as an adderor summer when a logarithmic signal is developed in the main IF channel12 as well as in the auxiliary channel 10. The digital signal is appliedfrom the A to D converter 66 through the lead 68 to a linear to logconverter 138 which may be a read only memory (ROM) having digitalvalues stored at addresses responsive to each linear input level, as iswell known in the art. The logarithmic signal is then applied from theconverter 138 through a lead 140 to a combiner 142 which may be an adderof a conventional digital type, adding the two logarithmic signals toprovide reliable detection to the detector 82 as well as to the videoprocessor 84.

Another modification of the system of FIG. 1 is shown in FIG. 3 in whichthe signal is applied from the attenuator 44 to an envelope detector 63which then applies the signal to the analog to digital converter 66.

The envelope detector 63 may be a square low detector, a linear detectoror a logarithmic detector. The digital signal is then applied to thecombiner 142 so that it operates as a summer, and the restored signal isapplied to the detector 82.

The A to D converter or encoder 102 may be of any suitable typeresponsive to a number of voltage ranges such as illustrated in FIG. 4in which the analog input voltage on the lead is applied to a subtractoralso receiving a reference voltage of 5.2 volts. In the illustratedarrangement it is assumed that the log scale factor is l0 db per voltand 0 db is equal to 0.5 volts, 10 db is equal to 1.5 volts and 20 db isequal to 2.5 volts. As a result, the quantized outputs are determinedfrom the following voltage ranges:

5.2V (47 db) O00 5.2V 5.8V OOl 5.8V 6.4V OlO 6.4V 7.0V Oll 7.0V up 100The signal at the output of the subtractor 150 is then applied to thecompare circuit 152 to determine if the output is positive or negativeand if it is negative, the signal is applied on a lead 154 representing000 to a decode circuit 156. If the output of the circuit 150 ispositive, then an AND gate 158 responds to the positive condition togate the voltage signal to the subtractor 160 which subtracts another0.5 volts and applies the remainder to a compare circuit 164. If thecompare circuit determines if the signal is negative, a lead 166 appliesa signal to the decode circuit 156 representative of a 001 condition. Ifthe output of the circuit 160 is positive, an AND gate 168 applies thevoltage to a subtractor circuit 172 which again subtracts 0.5 volts. Ifthe remainder from the circuit 172 is negative as determined by acompare circuit 174, the signal representative of 010 condition isapplied to a lead 176 to the decode circuit 156. If the remainder ispositive, an AND gate 180 applies the remainder to a subtractor circuit182 which again subtracts 0.5 volts therefrom. If the output from thecircuit 182 is negative, a compare circuit 184 applies a signalrepresentative of a 011 condition to a lead 186 which is in turn appliedto the decode circuit 156. If the signal developed by the subtractor 182is positive, AND gate 190 applies the voltage to a subtractor 192 whichagain subtracts 0.5 volts from the signal value and applies a signal toa compare circuit 196. If the difference is negative, the comparecircuit 196 applies a signal on a lead 198 representative of a 100condition to the decode circuit 156. The decode circuit 156 responds tothe true or false conditions of each of the leads 154, 166, 176, 186 and198 to apply the three binary signals to the encode unit 108, which mayin some systems include three flip flops. The subtraction and comparisonoperation of the circuit of FIG. 2 may be controlled in somearrangements in accordance with the invention with the clock pulsesutilized to rapidly set the flip flops in the unit 108 during each rangebin interval after the subtraction and comparison operation has beenperformed. In other arrangements, several range bin delays may beprovided in the A to D converter 102.

Referring now principally to FIG. 1, the decode logic unit 114 respondsto the input code by controlling the attenuators 40, 42 and 44respectively represented by A, B and C in accordance with the followingtable:

A/D Output lOO Thus it can be seen that each required attenuationcondition is provided by the three series coupled attenuators of theattentuation unit 39.

To further describe the encode unit 108, if the signal at the output ofthe A to D converter 102 is a binary number and the recombiner is asummer (logarithmic signals in both channels) the encoder 108 stores thenumber 1 range bin period and passes it on to the delay circuit 122.lfthe signal is linear in'the main channel 12 then the encode unit mayprovide a decoded output or multiplier value to the multiplier 72 inaccordance with the following table:

Attenuation Decode Multiplication Value 1 DUI 6db 2 010 12 db 4 I00 24db16 It is to be noted that the above table provides multiplication bydigital multiples of two so that the multiplier 72 may be a shiftregister requiring a shift to the left for each power of 2, as is wellknown in the art. When the summer 142 of FIGS. 2 and 3 is utilized, theencoder 108 may be required to include a scale factor to bring thesignal back to its original amplitude, on a range bin by range binbasis.

Referring now to the waveforms of FIG. 5, a waveform 210 shows the inputsignal at IF frequency relative to the 47 db reference of the mainchannel 12 and within the limitations of the 74 db illustrated dynamicrange of the auxiliary channel 10. The operation is performed duringeach of range bins 1 to it. Although for convenience of illustrationonly a limited number of range bins are shown, it is to be understoodthat systems of any desired number of range bins may be utilized such as200 to a thousand, for example. The attenuated signal on the lead 64 isshown by a waveform 212 having all portions of the signal of thewaveform 210 that is above the 47 db amplitude level attenuated to or asmall increment below that level. Other portions of the signal of thewaveform 210, that are below the 47 db level, are not attenuated oraffected by the dynamic gain control of the invention. A delay 214 isillustrated representative of the IF delay in the unit 37 and may be thedelay between the lead 96 and the output leads of the decode logiccircuit 114. The signal is then applied through the lead 67 and as adigital signal to the recombiner 72, with the signal of a waveform 218representing the recombined signal on the lead 80 in analog form forillustrative purposes. A delay 220 is provided representative of thedifference between the delay in the unit 122 and any delay in the encodeunit 108 relative to the entire delay in the main channel 12 whichincludes the delay in the delay line unit 37. The delay unit 122 may beequal to the delay between the leads 64 and the recombiner 72 when anMTI unit is not included in the system. It is to be noted that allportions of the signal of the waveform 212 that have been attenuated,have a signal of amplitude value equal to the amount of attenuationadded thereto or combine by multiplying, to provide the reformed signalhaving substantially all the amplitude characteristics of the originalsignal as shown by the recombined signal of the waveform 218.

Referring now to FIG. 6 as well as to FIG. 1, additional waveforms areshown to illustrate the attenuation and recombining operation inresponse to a specific signal return over range bins I10 to 119, forexample. For illustrative purposes the system delays as describedrelative to FIG. 5 are not shown in FIG. 6 and it is assumed that theattenuation and recombining occurs during corresponding range bins. Thesignal of the waveform 230 shows the IF signal in the lead 38 relativeto a level 231 which may represent an analog to digital maximum responselevel of the A/D unit 66. During range bins 114 and 116, the signalamplitude exceeds the dynamic rangeof the converter 6.6 and the.

information would be lost by clipping if it were not for the system ofthe invention. At the output of the attenuator 39, the signal of thewaveform 232 is attenuated to be below the level 231 so that clipping isnot provided but with the result that all the amplitude information isnot available. In response to the encode unit 108 and the recombiners 72or 142 of FIGS. 2 and 3, the signal of the waveform 234 is provided onthe lead by adding in the proper signal voltage during range bins 114and 116. Thus it can be seen that by the signal of the waveform 234 allthe target and signal amplitude characteristics are returned so thatdetection and video processing can be performed in a highly reliablemanner.

Thus, there has been described an improved automatic dynamic rangecontrol system that is applicable in radar systems utilizing MTI ormonopulse type processing, for example. The system allows substantiallyinstantaneous or real type automatic gain control so as to overcome theundesired effects of system dynamic range limitations, while at the sametime restoring the signal to substantially its original condition forhighly reliable signal detection and video processing. Thus, aninstantaneous automatic gain control system is provided that does noteliminate the signal modulation detail as in conventional systems. Theuse of an auxiliary large dynamic range logarithmic channel formeasurement purposes allows the gain control function to be appliedaccurately to the main channel. The system has been found to operatereliably by utilizing a relatively large increment of quantizingaccuracy.

What is claimed is: 1. A gain control system responsive to a radarsignal having a predetermined range of signal amplitude comprising aradar channel responsive to said radar signal and having a predetermineddynamic range limitation,

an auxiliary channel responsive to said radar signal and having adynamic range at least equal to said predetermined range of signalamplitude,

decoding means included in said auxiliary channel for developing codedsignals representative of the signal amplitude in excess of the dynamicrange of said radar channel,

attenuating means included in said radar channel and responsive to saiddecoding means to attenuate the radar signal amplitude when it exceedsthe dynamic range of the radar channel,

and recombining means included in said radar channel subsequent to saidattenuating means and responsive to said decoding means to restore theattenuated amplitude to said radar signal.

2. The system of claim 1 in which said auxiliary channel includes asequential detector providing logarithmic amplification.

3. The system of claim 2 in which said radar channel and said auxiliarychannel respectively provide a linear signal and a logarithmic signal tosaid recombining means and said recombining means is a multiplier.

4. The system of claim 3 in which said recombining means includesencoding means for converting the coded signals to multiplying factors.

5. The system of claim 1 in which said radar channel and said auxiliarychannel both provide logarithmic signals to said recombining means andsaid recombining means is a summing unit.

6. A dynamic range gain control system operative in a radar systemhaving at least one main channel that has a dynamic range limitation,said radar system providing radar signals during each of a plurality ofrange bins to said main channel of a predetermined dynamic range offluctuation comprising an auxiliary channel responsive to said radarsignal and including logarithmic amplifier means having a dynamic rangeat least equal to the predetermined range of said radar signal,

encoding means coupled to said logarithmic amplifier means fordeveloping coded signals representative of predetermined ranges ofamplitude of the radar signal above the dynamic range limitation of saidmain channel,

attenuating means included in said main channel and responsive to saidcoded signals during each range bin to control the amplitude of saidradar signal, and recombining means included in said main channel at aselected position subsequent to said attenuating means and coupled tosaid encoding means for increasing the amplitude of said radar signalduring each range bin corresponding to the range bin that was attenuatedby said attenuating means.

7. The system of claim 6 in which an analog to digital converter iscoupled between said attenuating means and said recombining means.

8. The system of claim 7 in which the signals in said auxiliary channeland said main channel respectively apply logarithmic and linear signalsto said recombining means and said recombining means is a multiplier.

9. The system of claim 7 in which the signals in said auxiliary channeland in said main channel applied to said recombining means are bothlogarithmic and said recombining means is a summer.

10. The system of claim 8 further including multiplier coding meanscoupled between said encoding means and said recombining means.

1. A gain control system responsive to a radar signal having a predetermined range of signal amplitude comprising a radar channel responsive to said radar signal and having a predetermined dynamic range limitation, an auxiliary channel responsive to said radar signal and having a dynamic range at least equal to said predetermined range of signal amplitude, decoding means included in said auxiliary channel for developing coded signals representative of the signal amplitude in excess of the dynamic range of said radar channel, attenuating means included in said radar channel and responsive to said decoding means to attenuate the radar signal amplitude when it exceeds the dynamic range of the radar channel, and recombining means included in said radar channel subsequent to said attenuating means and responsive to said decoding means to restore the attenuated amplitude to said radar signal.
 2. The system of claim 1 in which said auxiliary channel includes a sequential detector providing logarithmic amplification.
 3. The system of claim 2 in which said radar channel and said auxiliary channel respectively provide a linear signal and a logarithmic signal to said recombining means and said recombining means is a multiplier.
 4. The system of claim 3 in which said recombining means includes encoding means for converting the coded signals to multiplying factors.
 5. The system of claim 1 in which said radar channel and said auxiliary channel both provide logarithmic signals to said recombining means and said recombining means is a summing unit.
 6. A dynamic range gain control system operative in a radar system having at least one main channel that has a dynamic range limitation, said radar system providing radar signals during each of a plurality of range bins to said main channel of a predetermined dynamic range of fluctuation comprising an auxiliary channel responsive to said radar signal and including logarithmic amplifier means having a dynamic range at least equal to the predetermined range of said radar signal, encoding means coupled to said logarithmic amplifier means for developing coded signals representative of predetermined ranges of amplitude of the radar signal above the dynamic range limitation of said main channel, attenuating means included in said main channel and responsive to said coded signals during each range bin to control the amplitude of said radar signal, and recombining means included in said main channel at a selected position subsequent to said attenuating means and coupled to said encoding means for increasing the amplitude of said radar signal during each range bin corresponding to the range bin that was attenuated by said attenuating means.
 7. The system of claim 6 in which an analog to digital converter is coupled between said attenuating means and said recombining means.
 8. The system of claim 7 in which the signals in said auxiliary channel and said main channel respectively apply logarithmic and linear signals to said recombining means and said recombining means is a multiplier.
 9. The system of claim 7 in which the signals in said auxiliary channel and in said main channel applied to said recombining means are both logarithmic and said recombining means is a summer.
 10. The system of claim 8 further including multiplier coding means coupled between said encoding means and said recombining means. 